Add developer documentation for new compositional pipeline code

[pokey]

• [ ] Video
• [ ] Developer overview in contributor docs
• [ ] Diagram
• [ ] Docstrings on things like Target, BaseTarget, etc
• [ ] Add document on difference between weak, raw, plain, and token targets
• [ ] Glossary, including at least:
  • [ ] domain
  • [ ] removalRange
  • [ ] contentRange
  • [ ] iterationScope
  • [ ] boundary
  • [ ] interior
  • [ ] target